1. Field of the Invention
The subject invention pertains to computer systems, and more particularly, to an encoder and decoder for a computer system for encoding characters in a message prior to transmission from a first data processing system to a second data processing system thereby reducing the bandwidth or message length of the transmitted characters and increasing the transmission rate; and for decoding the encoded characters when the encoded characters are received by the second data processing system.
2. Description of the Prior Art
In computer systems, when a message is transmitted from a first data processing system to a second data processing system, the characters of the message are examined carefully by the first data processing system prior to transmission. For example, a computer terminal communicates with an attached telephone line through the use of a modem. A computer program stored in the modem examines each character of the message prior to transmission. Certain characters are reserved for specific control functions in the computer system, such as line feed and carriage return. When these certain characters are encountered, the computer program in the modem performs the particular control function. Therefore, if one or more of the certain characters appear as characters in the message to be transmitted, these certain characters in the message are unacceptable for transmission and must be converted to other characters which are acceptable for transmission, the conversion being done prior to transmission of the message by the modem from the computer terminal to the telephone line.
In the prior art, in order to avoid transmitting unacceptable characters, each original character in the message to be transmitted was duplicated such that two characters, corresponding to each original character, were transmitted. Using this prior art technique, the original message to be transmitted doubled in length. In addition, on the receiving end, since each original character of the message was duplicated, the second data processing system did not mis-interpret the meaning of the duplicated unacceptable characters; however, since the original message length was doubled, the transmission rate of the original message was cut approximately in half. The first data processing system (the modem and computer terminal) is capable of transmitting messages at a rate of 30 characters/second, but, using the prior art encoding technique, the first data processing system transmitted at a rate of 15 characters/second.